I don’t know much about highway construction materials in Estonia, but here in the states the concrete sections will vary between 8-14 inches with rebar for additional structural load capacity. The base material under the concrete can be 4 inches of asphalt over another 12 inches of compacted base rock. Most highways will have some combination of the above depending on the geotechnical characteristics of the site.
And the concrete may vary between 4,000 and 6,000psi in compressive strength.
Oil shale is a big thing in Estonia, and one way they have gotten energy from it is to grind it up and burn it directly. I have heard that the baked gravel left over has some uses as a construction material. The baking probably metamorphoses it into something that is better as construction material than raw shale gravel.